Class-D Amplifier Common-Mode Compensation for Stable Loop Gain
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Solution Overview
Problem
Class-D amplifiers using BD modulation switching techniques experience common-mode voltage fluctuation, which degrades the gain of differential operational amplifiers within the loop filter.
Innovation Solution
A common-mode voltage compensation circuit is integrated into the amplification circuit, utilizing a switching-type digital-to-analog converter with compensation resistors and switches to stabilize the common-mode voltage, ensuring it remains at the ideal value of VDD/2, thereby eliminating fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If BD modulation switching technique is used to achieve high-efficiency amplification, then power loss is significantly reduced, but common-mode voltage fluctuation occurs which degrades the gain of differential operational amplifier
Solution Approach 1:
A common-mode voltage compensation circuit is introduced as an intermediary component between the power driver output and the loop filter input. This compensation circuit includes compensation resistors (RCMp, RCMn) coupled to a control node that switches between VDD and GND based on the output signal states, thereby mediating the common-mode voltage to maintain stable differential input voltages for the operational amplifier and eliminating gain degradation while preserving the high-efficiency BD modulation switching operation
2Reliability
If common-mode voltage compensation circuit is added to eliminate fluctuation, then gain stability is improved, but device complexity increases
Solution Approach 1:
The common-mode voltage compensation function is extracted as a separate, dedicated circuit module with clearly defined components (compensation resistors RCMp/RCMn, switching elements, and control logic). By isolating this compensation function from the main signal path components, the circuit achieves stable gain without requiring fundamental redesign of the entire amplifier architecture, thus limiting the increase in overall device complexity
Solution Approach 2:
The compensation circuit operates by dynamically changing the common-mode voltage parameter (switching between VDD and GND levels) based on the output signal states. This parameter change approach allows the circuit to maintain stable differential voltages across varying operating conditions without requiring complex active compensation components, thereby achieving gain stability with relatively simple circuitry
Data Source
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AI summary
An amplification circuit with a common-mode voltage compensation circuit is shown. The common-mode voltage compensation circuit has a first compensation resistor coupled between an input terminal of a loop filter of the amplification circuit and a control node, and a second compensation resistor coupled between another input terminal of the loop filter and the control node. The control node is coupled to a power ground voltage when the two output signals of the amplification circuit are high, and it is coupled to a power supply voltage when the two output signals of the amplification circuit are low.